Pb, which is ordinarily added to the copper-alloy for sliding use, expands and is elongated on the sliding surface upon temperature rise during the sliding. As a result, because Pb cools the sliding surface and simultaneously realizes its excellent self-lubricating properties, seizure is, consequently, prevented. In addition, since Pb forms a soft dispersing phase, Pb has conformability and such property that foreign matters are embedded in Pb.
However, Pb is liable to be corroded by acids other than sulfuric acid. When Pb is present in the form of coarse particles in the Cu alloy, the load ability of a bearing is lowered. Therefore, Patent Document 1 (Japanese Examined Patent Publication (kokoku) Hei 8-19945) proposes to disperse Pb in the form of fine particles expressed by a particular calculation equation. The equation can be interpreted to mean the following. The total Pb particles in the visual field of 0.1 mm2 (105 μm2) are observed. The average area ratio of these particles is converted to one particle, which is 0.1% or less. According to an example of this publication, a Cu—Pb—Sn pre-alloy powder is used. In addition, it is described that a finer Pb structure is obtained at lower sintering temperature. It is, therefore, understood that the technique employed in this publication is to suppress the precipitation and growth of Pb by low-temperature sintering.
It is known from Patent Document 2 (Japanese Examined Patent Publication (kokoku) No. Hei 7-9046 that, in order to enhance the wear resistance of the sintered copper alloy, such carbides as Cr2C3, Mo2C, WC, VC and NbC are added in the sintered copper alloy as hard matters. According to this publication, the copper-alloy powder having from 10 to 100 μm of average particle-diameter and the hard-matter powder having 5 to 150 μm of the average particle-diameter are mixed by a V type blender, followed by compacting and sintering. The description that Pb is present in the grain boundaries of the copper particles (column 4, lines 21 through 22) is not inconsistent with knowledge derived from an equilibrium phase diagram, that is, Pb is hardly not dissolved in the solid Cu.
A Pb-free alloy, which attains the sliding properties equivalent to the Cu—Pb based sintered alloy, is known from Patent Document 3 (Japanese Unexamined Patent Publication (kokai) No. Hei 10-330868. It is apparent from the drawings of this publication that the location of the Bi (alloy) phase is the grain-boundary triple points and the grain boundaries in the vicinity to the triple points.
It is proposed in Patent Document 4 (Japanese Patent No. 3421724) that the hard matters incorporated into the Pb or Bi phase prevent Pb and Bi from flowing out of the sintered copper alloy; the Pb or Bi phase behaves as a cushion of the hard matters, the attacking property of which against the opposite shaft is mitigated; the separated hard matters are again captured by the Pb or Bi phase, thereby mitigating abrasive wear. In this patent, the presence of the hard matters is such that they are enveloped in the Bi phase. The dimension of the Bi phase is, therefore, larger than that of the hard matters.
It is disclosed in Patent Document 5 (Japanese Unexamined Patent Publication (kokai) No. 2001-220630) that an intermetallic compound is added to enhance the wear resistance of the Cu—Bi(Pb) based sintered alloy; its micro-structure is such that the intermetallic compound is present around the Bi or Pb phase. During sliding, the intermetallic compound is convex and the Bi or Pb phase as well as the Cu matrix are concave on the surface of copper alloy to form oil reserving portions. As a result, seizure resistance and fatigue resistance of the sliding material are improved. An example of the sintering condition proposed is from 800 to 920° C. for approximately 15 minutes.
Patent Document 1: Japanese Examined Patent Publication (kokoku) No. Hei 8-19945
Patent Document 2: Japanese Examined Patent Publication (kokoku) No. Hei 7-9046
Patent Document 3: Japanese Unexamined Patent Publication (kokai) No. Hei 10-330868
Patent Document 4: Japanese Patent No. 3421724
Patent Document 5: Japanese Unexamined Patent Publication (kokai) No. 2001-220630
Patent Document 6: Japanese Unexamined Patent Publication (kokai) No. 2002-12902